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Variation in speed of light - how does it accelerate after being slowed down?

  1. Feb 19, 2012 #1
    As you will see this is my first post so I apologize if I have chosen the wrong sub-forum. My academic training (maths) was many years ago and it now seems to me that for the last 30 or so years my brain has been in a kind of intellectual limbo from which it is only now beginning to emerge, so please be gentle... I think this has been driven by the need to try to explain the world to my children, and I only wish I had had my current level of intellectual curiosity when I was a student - I realise now just how much I wasted the opportunities offered during that period of my life. Oh well...

    My first question relates to what I believe is an established fact that the speed of light can appear to vary (or actually does vary) as it passes through different media. In particular, it can be slowed down by any media other than vacuum. My specific question is this; if light can be slowed down as it passes from vacuum into a particular type of media how does it then speed up again as it emerges from that media back into vacuum? By my simple understanding this means it must be acquiring energy from somewhere, so it can accelerate, but if so where? Or is my understanding fundamentally flawed?

    And if light can be slowed down, how slow can it be made to go? Could we ever perceive with our own senses (ie without instrumentation) a delay between light entering transparent object and the same light exiting the other side?

    Any insights will be gratefully received, thanks. And I have many more follow up questions if anyone cares to enter a dialogue with me.

  2. jcsd
  3. Feb 19, 2012 #2


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    In the strict sense, it doesn't. What happens to light passing through, say, glass, is that it travels at "c" from one atom to another where it is absorbed, then re-emitted. The light itself always travels at "c"- it is the time take to absorb, then re-emit that makes it appear that light slows down in glass.
  4. Feb 19, 2012 #3

    George Jones

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    A more detailed version of HallsofIvy's answer:

    https://www.physicsforums.com/showthread.php?t=511177 [Broken].
    Last edited by a moderator: May 5, 2017
  5. Feb 19, 2012 #4


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    Not so much "more detailed" as saying I am wrong- it is not the atoms that absorb the photons, it is the lattice of atoms. Thanks, that clears up some questions I had about that.
  6. Feb 19, 2012 #5
    Thanks to both for your responses, and thanks also for not telling me off for not finding that post myself:redface:

    So that does pretty much answer my first question. Light itself does not slow down, it just gets "interfered with" on its way through the medium. Hence there is no acceleration once it leaves the medium as it never actually slowed down in the first place.

    The article says where a photon is able to pass through a medium this is because "the lattice does not absorb this photon and it is re-emitted but with a very slight delay".

    Does this mean it is literally the same photon from entry to emergence, or is a new photon generated at each interaction with the lattice? Or is the distinction meaningless?

    Either way is it fair to say this ion / electron lattice is a kind of photon capacitor, in that it temporarily stores the light energy from the incoming photon before releasing it again (possibly as a new photon)? This begs the question, if the medium contains a high enough density of the right kind of lattice, could the medium actually store light and then emit it a measurable period of time later? I'm thinking of something like a glowing radium dial on a watch - or is that exactly what a radium dial is doing when it glows for a period of time after being charged by exposure to light?
  7. Feb 19, 2012 #6
    According to my textbook (Fundamentals of Optics, Jenkins&White) that is also not entirely correct (probably the FAQ needs a little rework). Refraction is there explained with scattering theory. As the primary wave traverses the solid at speed c, it brings the charges in vibration and this forced oscillation emits secondary waves with a slight phase delay. The resulting wave is then also phase delayed. In this description there is no absorption or re-emission of waves (although there is, obvioulsy, transfer of energy in and out).

    PS @childej: I think that this precision answers all your later questions.

    Last edited: Feb 19, 2012
  8. Feb 21, 2012 #7
    Harald, thanks for the clarification. I'm not clear on the distinction between absorption and re-emission by the lattice, and causing a variation in vibration leading to the emission of a new wave. However the key point remains that, as in my previous post, light itself does not slow down, it is simply held up by its interaction with the medium it is passing through.

    I think I have answered for myself my questions as to whether it is theoretically possible to create a material that would store light for useful amount of time.

    My old analogue watch had a dial which, after exposure to sunlight, would glow in the dark for a period of time. I think this is an example of Phosphorescence which according to Wikipedia is "fluorescence slightly delayed after initial absorption of radiation (on a scale of seconds to hours)". However I am not sure this is the same as what we have been discussing as I associate phosphorescence with light being emitted back the way it came, rather than simply being a slowing down and storing of the light entering the medium. Are they the same thing?
  9. Feb 21, 2012 #8
    My fault. I should probably have written "not absorption and subsequent re-emission". As I now understand it, the secondary wave or wavelet emission is simultaneous (or practically simultaneous?) with the forced motion of the electrons, and thus also simultaneous with the temporary transformation of radiation energy to kinetic energy and back.
    Yes indeed; and I think that your expression "it just gets interfered with" hits the nail on the head.
    It is very different. Phosphorescence implies true absorption of radiation: the energy is stored for some time in the molecules, before it is given back. However, it is possible to produce refractive materials that slow down light very much. For a discussion, see:
  10. Feb 26, 2012 #9

    Thanks again. Interestingly the link you provided is one my son referred to when we discussed the same question. So now I know where he got it from.
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